With the rapid growth of emerging bandwidth-demanding network services, next-generation dense wavelength division multiplexed (DWDM) optical transport technologies employing multilevel modulation formats are highly desirable to deliver information bits as many as possible over existing band-limited ITU-T channels. Polarization multiplexing return-to-zero quadrature phase shift keying (PM-RZ-QPSK) with digital coherent detection has been recognized as the next optical transport network standard that mitigates optical link impairments by multiplexing data tributaries at a lower bit-rate so as to be handled easily by DSP-orientated coherent receivers.
Currently, the existing 50 GHz DWDM channel spacing can barely accommodate a 112 Gb/s PM-QPSK signal. But since the line rate for the new-generation PM-QPSK products may go up to 128 Gb/s due to the use of soft-decision forward error correction (FEC) with a higher overhead, there could be significant penalties resulting from the insufficient channel bandwidth, which will be even more problematic when considering the bandwidth narrowing effect caused by a series of in-line optical filters such as reconfigurable optical add-drop multiplexer (ROADM) along the optical transmission path.
Return-to-zero (RZ) pulse is usually more tolerant to filtering and nonlinear degradations than non return-to-zero (NRZ) pulse. But if the channel spacing is 25 GHz or below for supporting future terabit Nyquist-WDM superchannel, the conventional RZ pulse will not function well when combining ten 128 Gb/s subchannels under such aggressive optical filtering.